Optical method for the characterization of laterally patterned samples in integrated circuits

ABSTRACT

Disclosed is a method for characterizing a sample having a structure disposed on or within the sample, comprising the steps of applying a first pulse of light to a surface of the sample for creating a propagating strain pulse in the sample, applying a second pulse of light to the surface so that the second pulse of light interacts with the propagating strain pulse in the sample, sensing from a reflection of the second pulse a change in optical response of the sample, and relating a time of occurrence of the change in optical response to at least one dimension of the structure.

CROSS-REFERENCE TO A RELATED APPLICATION

This patent application is related to co-pending application Ser. No.08/954,347, filed Oct. 17, 1997, which is a division of application Ser.No. 08/689,287, filed Aug. 6, 1996, which issued as U.S. Pat. No.5,748,318. The present application is also related to co-pendingapplication Ser. No. 09/111,456, filed on Jul. 7, 1997. The disclosureof each related application is incorporated by reference in its entiretyinsofar as it does not conflict with the teachings of the presentinvention.

This invention was made with government support under grant numberDEFG02-ER45267 awarded by the Department of Energy. The government hascertain rights in the invention.

FIELD OF THE INVENTION

This invention relates generally to optical metrology methods andapparatus and, more particularly, to optical techniques that usepicosecond scale light pulses for characterizing samples.

BACKGROUND OF THE INVENTION

Currently, in the semiconductor industry there is a great interest inthe characterization of thin films and small structures. Integratedcircuits are made up of a large number of patterned thin films depositedonto a semiconductor substrate, such as silicon. The thin films includemetals to make connections between the transistors making up the chip,and insulating films to provide insulation between the metal layers(see: S. A. Campbell, The Science and Engineering of MicroelectronicFabrication, Oxford University Press, (1996)). The metal films(interconnects) are typically arranged as a series of patterned layers.At the present time there may be 4 or 5 layers of interconnects. It islikely that as more complex integrated circuits are developed, requiringa greater number of interconnections, the number of layers willincrease. Metals of current interest include, for example, aluminum,copper, titanium and silicides. Insulating films include, for example,oxide glasses of various compositions and polymers. The films may bepatterned so as to form wires running across the surface of the sample.For convenience, each such wire shall be referred to as a structure.These wires may be embedded into a film of another material or may bedeposited on top of another film. For some samples of interest, all ofthe wires have the same nominal dimensions, run in the same directionacross the surface of the sample, and are equally spaced. If the wiresrun in the direction parallel to the z-axis, for example, the geometryof the sample is entirely specified when the cross-section in the x-yplane is determined (see FIG. 1). For this reason such samples arereferred to as two-dimensional patterned structures. Another type ofsample of interest might include a two-dimensional array of identicalrectangular parallelepipeds disposed on a surface (see. FIG. 2). Thegeometry of such a sample cannot be completely specified by determiningthe geometry in a single x-y plane. For this reason, a sample of thistype is referred to as a three-dimensional patterned sample. Othersamples might still be periodic, but with a more complicated pattern.For example, the sample could be made up of a sequence ABABAB, of wireswith two different dimensions, such that wire A has width a_(A) andheight b_(A), and wire B has width a_(B) and height b_(B). Alternately,the sample could include a sequence of wires, all of which have the samegeometry, but the spacing between the wires could alternate between thevalues c₁ and c₂.

In the production of integrated circuits it is essential that allaspects of the fabrication process be controlled as closely as possible.It is important to measure the geometry of the sample, i.e., thethickness of thin films, the lateral dimensions of wire structures suchas the dimensions a, and b in FIG. 1, the spacing c between structures,etc. It is also important to be able to measure mechanical andelectrical properties of the structure, such as the adhesion between awire and the film it is in contact with.

There are a number of techniques currently available for thedetermination of the geometry of such samples. These include:

(1) Scanning Electron Microscopy. In this technique an electron beam isfocused onto a small spot on the sample, and electrons that arescattered from the sample surface are detected. The electron beam isscanned across the surface of the sample, and an image of the samplesurface is obtained. For a two-dimensionally patterned sample thistechnique can determine the dimensions a and c as shown in FIG. 1. For athree-dimensionally patterned sample the dimensions a₁, a₂, c₁, and c₂of FIG. 2 can be determined. This method cannot be used to determine thedimension b of FIG. 1. In addition, the method is time consuming sincethe sample must be placed into the high vacuum chamber of the electronmicroscope. In addition, to measure dimensions with scanning electronmicroscopy it is necessary to perform a careful calibration of theinstrument.

(2) Scanning Electron Microscopy with Sectioning. In this technique,material is removed from the sample to expose a section of the samplelying in the xy-plane. Scanning electron microscopy is then used to viewthis section of the sample. This method is thus able to measure thedimension b shown in FIGS. 1 and 2. This method has the followingdisadvantages: i) A considerable amount of time is required to preparethe sample. ii) The sample has to be destroyed in order to make themeasurement. iii) The method is time-consuming since the sample has tobe transferred into the high-vacuum chamber of the electron microscopein order for the measurement to be made.

(3) Atomic Force Microscopy. In this technique an atomic forcemicroscope is used instead of an electron microscope to view the surfaceof the sample. The top surface of the sample can be viewed directly, asin (1) above, and measurements can also be made after sectioning thesample as in method (2). This method has the disadvantage that aconsiderable amount of time is involved for the measurements to be made.In addition, if the sample is sectioned, it is destroyed.

OBJECTS OF THE INVENTION

It is a first object of this invention to provide a method for the rapiddetermination of the dimensions of samples composed of one or morestructures, or a periodic array of structures, deposited directly onto asubstrate, or onto a film deposited on a substrate, or embedded within afilm or within the substrate.

It is a second object of this invention to provide a method that doesnot require the destruction of such samples.

It is a further object of this invention to determine mechanical andelectrical properties of such samples.

SUMMARY OF THE INVENTION

In accordance with a first method of the present invention, a method isprovided for characterizing a sample having a structure that is disposedon or within the sample. The method comprises the steps of applying afirst pulse of light to a surface of the sample for creating apropagating strain pulse in the sample, applying a second pulse of lightto the surface so that the second pulse of light interacts with thepropagating strain pulse in the sample, sensing from a reflection of thesecond pulse a change in optical response of the sample, and relating atime of occurrence of the change in optical response to a dimension ofthe structure.

In accordance with a second method of the present invention, a method isprovided for characterizing a sample having a structure that is disposedon or within the sample. The method comprises the steps of applying afirst pulse of light to a surface of the sample to excite the structureinto a normal mode of vibration, applying a second pulse of light to thesurface, sensing from a reflection of the second pulse a change inoptical response of the sample, relating the change in optical responseto an oscillatory component of the vibration; and relating theoscillatory component to a spatial or electrical characteristic of thestructure.

In accordance with a first embodiment of the present invention, anon-destructive system is provided for characterizing a sample having astructure that is disposed on or within the sample. The system comprisesan optical beam generator for applying a first pulse of light to asurface of the sample for creating a propagating strain pulse in thesample, an optical beam generator for applying a second pulse of lightto the surface so that the second pulse of light interacts with thepropagating strain pulse in the sample, a sensor for sensing from areflection of the second pulse a change in optical response of thesample, and a processor for relating a time of occurrence of the changein optical response to a dimension of the structure.

In accordance with a second embodiment of the present invention, anon-destructive system is provided for characterizing a sample having astructure that is disposed on or within the sample. The system comprisesan optical beam generator for applying a first pulse of light to asurface of the sample to excite the structure into a normal mode ofvibration, an optical beam generator for applying a second pulse oflight to the surface, a sensor for sensing from a reflection of thesecond pulse a change in optical response of the sample, a processor forrelating the change in optical response to an oscillatory component ofthe vibration, and a processor for relating the oscillatory component toa spatial or electrical characteristic of the structure.

BRIEF DESCRIPTION OF THE DRAWINGS

The above set forth and other features of the invention are made moreapparent in the ensuing Detailed Description of the Invention when readin conjunction with the attached Drawings, wherein:

FIG. 1 is an illustration of a two-dimensional patterned structure;

FIG. 2 is an illustration of a three-dimensional patterned sample, andmore specifically, a two-dimensional array of elements disposed on asurface;

FIG. 3 is a block diagram of one embodiment of an ultra-fast opticalsystem that is suitable for use in practicing this invention,specifically, a parallel, oblique beam embodiment;

FIG. 4 illustrates a portion of FIG. 3 in greater detail;

FIG. 5 is a flowchart of a method for characterizing a sample byevaluating an oscillatory component of the sample in accordance with thepresent invention; and

FIG. 6 is another embodiment of an ultra-fast optical system that issuitable for use in practicing this invention, specifically, a normalpump, oblique probe embodiment;

FIG. 7 is a block diagram of an other embodiment of an ultra-fastoptical system that is suitable for use in practicing this invention,specifically, a single wavelength, normal pump, oblique probe, combinedellipsometer embodiment;

FIG. 8 is a block diagram of another embodiment of an ultra-fast opticalsystem that is suitable for use in practicing this invention,specifically, a dual wavelength, normal pump, oblique probe, combinedellipsometer embodiment;

FIG. 9 is a block diagram of another embodiment of an ultra-fast opticalsystem that is suitable for use in practicing this invention,specifically, a dual wavelength, normal incidence pump and probe,combined ellipsometer embodiment;

FIG. 10 illustrates a timed sequence of a plurality of consecutive pumppulses and corresponding probe pulses;

FIG. 11 illustrates the operation of a transient grating embodiment ofthis invention, wherein the pump pulse is divided and made to interfereconstructively and destructively at the surface of the sample;

FIG. 12 illustrates a pulse train of pump beam pulses having anoverlying low frequency intensity modulation impressed thereon;

FIG. 13 illustrates a further embodiment wherein one or more opticalfibers are positioned for delivering the pump beam and/or probe beam andfor conveying away the reflected probe beam;

FIG. 14 is a side view a terminal end of a fiber optic that has beenreduced in cross-sectional area for delivering an optical pulse to asmall surface area of a sample;

FIG. 15 is a sectional view of a two-dimensionally patterned samplecomposed of an array of wires each with rectangular cross-sectionembedded in a substrate;

FIG. 16 is a sectional view of a two-dimensionally patterned samplecomposed of an array of wires each with angled side walls embedded in asubstrate where a pump beam and a probe beam are applied at obliqueincidence;

FIG. 17 is a perspective view of a two-dimensionally patterned samplecomposed of an array of wires embedded in a substrate, where each wirehas a coating on its sides, and where a pump beam is applied at obliqueincidence;

FIG. 18 is a perspective view of a two-dimensionally patterned samplecomposed of an array of wires embedded in a substrate, where each wirehas a coating on its sides, and where a pump beam is applied at normalincidence;

FIG. 19 is a flowchart of a method for characterizing a sample byrelating a change in optical response to a dimension of the structure inaccordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The teaching of this invention is practiced with an optical generatorand a detector of a stress wave within a sample. The sample is comprisedof a substrate having a structure, or an array of similar, but notnecessarily identical, structures deposited on it. These structures maybe located directly on top of the substrate, or may be embedded in thesubstrate, or may be deposited on top of one of a film, or films,deposited on the substrate, or may be embedded in one of these films.The structures may be composed of a single material, such as copper oraluminum or silicon, or may be composed of more than one material. Thestructures may be in the form of wires running in a particular directionacross the sample; this type of sample is referred to as atwo-dimensionally patterned sample (2D), or as a sample composed of wirestructures. An example of this type of sample is shown in FIG. 1. Notethat although the length of the wires is finite, this length is assumedto be much larger than the spacing between the wires, or the height ofthe wires (dimension b). The sample may also include an array ofelements forming a grid pattern on the surface of the substrate or film.This is referred to as a three-dimensionally patterned sample (3D) or asa sample composed of dot structures. Again, this pattern is repeatedover an area with dimensions large compared to the dimensions of eachindividual structure. The lateral dimensions of each structure, e.g.,the width for a wire structure, could range from 30 Å to 10 microns, andthe height of the structure, e.g., the dimension b shown in FIG. 1 or 2could range from 30 Å to 10 microns.

In this system, a non-destructive first light pulse is directed onto thesample. This first light pulse, referred to hereafter as a pump beam, isabsorbed in a thin layer on the surface of the sample. According to theangle at which the pump beam is incident onto the surface of the sample,the material making up the structures, the films and the substrate, thelight may be mostly absorbed in the top of each structure, the sides ofthe structure, one of the films, or in the substrate itself. When thepump beam is absorbed, the temperature of the surface layer isincreased, and the layer tries to expand. This launches a strain pulsethat propagates through the sample. The direction in which the strainpulse propagates is determined by the orientation of the surface fromwhich the pulse originates. For example, if the structures have verticalside walls and light is absorbed in a thin layer adjacent to thesewalls, a strain pulse is generated that propagates in the directionparallel to the surface of the substrate. On the other hand if eachstructure has a flat top and light is absorbed in the region near tothis surface, the strain pulse propagates in the direction normal to theplane of the substrate. In many types of sample, strain pulses withappreciable amplitude are generated in a number of different regions ofthe sample, and propagate in different directions.

The strain pulses propagate through the structures, the film, or films,and in the substrate. When a strain pulse reaches an interface betweendissimilar materials, a fraction of the pulse is reflected and afraction is transmitted. There is thus a time-dependent strain withinthe sample. This strain results in a change in the optical constants,real and imaginary parts of the dielectric constant, of the samplematerial, as a consequence of a piezo-optic effect. In addition, thereis a change in geometry of the sample. For example, the width of thewire structures in a 2D sample, i.e., dimension a of FIG. 1, is affectedby the strain and varies with time. The change in the optical constantsand the change in the geometry results in a change ΔR(t) in the opticalreflectivity R of the sample. The time t here indicates the time thathas elapsed since the application of the pump pulse.

The fact that there is a change in geometry of the structure as a resultof the propagation of the strain pulses affects the choice ofwavelengths for the probe beam. For example, it is known, see U.Gerhardt, Physical Review, 172, p. 651-664, 1968, that for copper thechange in the optical constants in the wavelength range 7000-8000 Å whena strain is applied is very small. Thus for a planar copper film, it ishard to make measurements using a probe beam with a wavelength in thisrange. However, for a laterally-patterned sample, it has beendemonstrated that with a probe beam in this wavelength range there canbe a large change ΔR(t) in the optical reflectivity. It is believed thatthis large change comes about because the strain pulses propagating inthe sample result in a time-dependent change in the size of thestructures, and hence also in the spacing between them.

This change ΔR(t) is measured by means of a second light pulse directedat the sample. This second light pulse, referred to hereafter as a probebeam, is time-delayed relative to the pump beam. Properties of thesample are determined by analysis of the transient optical response,e.g., changes in the reflected probe beam intensity.

Reference is now made to FIG. 3 and FIG. 4 for illustrating a firstembodiment of an apparatus 100 suitable for practicing this invention.This embodiment is referred to as a parallel, oblique embodiment.

This embodiment includes an optical/heat source 120, which functions asa variable high density illuminator, and which provides illumination fora video camera 124 and a sample heat source for temperature-dependentmeasurements under computer control. An alternative heating methodemploys a resistive heater embedded in a sample stage 122. One advantageof the optical heater is that it makes-possible rapid sequentialmeasurements at different temperatures, or at one stabilizedtemperature.

The video camera 124 provides a displayed image for an operator, andfacilitates the set-up of the measurement system. Appropriate patternrecognition software can also be used for this purpose, therebyminimizing or eliminating operator involvement. BS5 is a broad band beamsplitter that directs video and a small amount of laser light to thevideo camera 124. The camera 124 and processor 101 can be used toautomatically position the pump and probe beams on a measurement site.

The sample stage 122 is preferably a multiple-degree of freedom stagethat is adjustable in height (global z-axis), position (global x andy-axes), and optionally tilt (φ), and allows motor controlledpositioning of a portion of the sample relative to the pump and probebeams. The global z-axis is used to translate the sample vertically intothe focus region of the plump and probe, the global x and y-axestranslate the sample parallel to the focal plane, and the tilt axesadjust the orientation of the stage 122 to establish a desired angle ofincidence for the probe beam. This is achieved via a first positionsensitive detector PSD1 and a signal processor 101, as shown in FIG. 3and described below.

In an alternative embodiment, the optical head may be moved relative toa stationary, tiltable stage 122′ (not shown). This is particularlyimportant for scanning large objects, such as 300 mm diameter wafers. Inthis embodiment the pump beam, probe beam, and video signal can bedelivered to or from a translatable head via optical fibers or fiberbundles.

A pump-probe beam splitter 126 splits an incident laser beam pulse,preferably of picosecond or shorter duration, into pump and probe beams,and includes a rotatable half-wave plate (WP1) that rotates thepolarization of the unsplit beam. WP1 is used in combination with apolarizing beam splitter PBS1 to effect a continuously variable splitbetween pump and probe power. This split may be controlled by thecomputer by means of a motor to achieve an optimal signal to noise ratiofor a particular sample. The appropriate split depends on factors suchas the reflectivity and roughness of the sample. Adjustment is effectedby having a motorized mount rotate WP1 under computer control.

A first acousto-optic modulator (AOM1) chops the pump beam at afrequency of about 1 MHz. A second acousto-optic modulator (AOM2) chopsthe probe beam at a frequency that differs by a small amount from thatof modulator AOM1. The use of AOM2 is optional in the system illustratedin FIG. 3. Optionally, the AOMs may be synchronized to a common clocksource, and may further be synchronized to the pulse repetition rate(PRR) of the laser that generates the pump and probe beams. Optionallyan electro-optic modulator can be used in place of acousto-opticmodulators AOM1 or AOM2.

A spatial filter 128 is used to preserve at its output a substantiallyinvariant probe beam profile, diameter, and propagation direction for aninput probe beam which may vary due to the action of the mechanicaldelay line shown as a retroreflector 129. The spatial filter 128includes a pair of apertures A1 and A2, and a pair of lenses L4 and L5.An alternative embodiment of the spatial filter incorporates an opticalfiber, as described above. If the profile of the probe beam coming fromthe mechanical delay line does not vary appreciably as theretroreflector 129 is moved, the spatial filter 128 can be omitted.

WP2 is a second adjustable halfwave plate which functions in a similarmanner with PBS2 to the WP1/PBS1 combination of the beam splitter 126. Apart of the probe beam passing through beam splitter PBS2 impinges on abeam block BB1. Beam splitter BS2 is used to direct a small fraction ofthe probe beam onto reference detector D2. The output of D2 is amplifiedand sent through a low pass filter 130A to give an electrical signalLF2, which is proportional to the average intensity of the incidentprobe beam.

The probe beam after passing through BS2 is focused onto the sample bylens L2. As shown in FIG. 4, after reflection from the sample the beamis collimated and after passing polarizer 131 is incident onphotodetector D1. From the output of D1 two electrical signals arederived. The first signal LF1 is obtained by passing the amplifiedoutput of D1 through a low pass filter 130B to give an electrical signalproportional to the average intensity of the incident probe beam. Thesecond signal HF1 is obtained by passing the amplified output of D1through a high pass filter 130C that passes the frequency of modulationused for AOM1.

The low frequency signals LF1 and LF2 can be used to determine thereflectivity of the sample, after allowance is made for fixed losses inboth optical paths. The signal LF2 and the average (dc) output ofdetector D4 give a measure of the intensity of the pump and probe beams.These signals are fed to a computer, for example, the signal processor101, which in turn controls motorized waveplates WP1 and WP2. Thecomputer is programmed to adjust these waveplates so as to give thedesired total optical power and pump/probe ratio for a sample exhibitinga particular reflectivity.

The linear polarizer 131 is employed to block scattered pump lightpolarization, and to pass the probe beam. The beam splitter BS1 is usedto direct a small part of the pump beam, and optionally a small part ofthe probe beam, onto first Position Sensitive Detector (PSD1) that isused for autofocusing, in conjunction with the processor 101 andmovements of the sample stage 122. The PSD1 is employed in combinationwith the processor 101 and the computer-controlled stage 122 (tilt andz-axis) to automatically focus the pump and probe beams onto the sampleto achieve a desired focusing condition.

The detector D1 may be used in common for reflectometry, ellipsometry,and transient optical embodiments of this invention. However, theresultant signal processing is different for each application. Fortransient optical measurements, the DC component of the signal issuppressed, such as by subtracting reference beam input D2, or part ofit as needed, to cancel the unmodulated part of D1, or by electricallyfiltering the output of D1 so as to suppress frequencies other than thatof the modulation. The small modulated part of the signal is thenamplified and stored. For ellipsometry, there is no small modulatedpart, rather the entire signal is sampled many times during eachrotation of a rotating compensator (see discussion of FIG. 6, below),and the resulting waveform is analyzed to yield the ellipsometricparameters. For reflectometry, the change in the intensity of the entireunmodulated probe beam due to the sample is determined by using the D1and D2 output signals (D2 measures a signal proportional to theintensity of the incident probe). Similarly, additional reflectometrydata can be obtained from the pump beam using detectors D3 and D4. Theanalysis of the reflectometry data from either or both beams may be usedto characterize the sample. The analysis can be performed by signalprocessor 101, or any suitable general-purpose computer. The use of twobeams is useful for improving resolution, and for resolving anyambiguities in the solution of the relevant equations.

A third beam splitter BS3 is used to direct a small fraction of the pumpbeam onto detector D4, which measures a signal proportional to theincident pump intensity. A fourth beam splitter BS4 is positioned so asto direct a small fraction of the pump beam onto detector D3, whichmeasures a signal proportional to the reflected pump intensity.

FIG. 6 illustrates a normal pump beam, oblique probe beam embodiment ofapparatus 102. Components labeled as in FIG. 4 function in a similarmanner, unless indicated differently below. In FIG. 6 there is providedthe above-mentioned rotating compensator 132, embodied as a linearquarter wave plate on a motorized rotational mount, and which forms aportion of an ellipsometer mode of the system. The plate is rotated inthe probe beam at a rate of, by example, a few tens of Hz tocontinuously vary the optical phase of the probe beam incident on thesample. The reflected light passes through an analyzer 134 and theintensity is measured and transferred to the processor 101 many timesduring each rotation. The signals are analyzed according to known typesof ellipsometry methods to determine the characteristics of the sample(transparent or semitransparent films). This allows the (pulsed) probebeam to be used to carry out ellipsometry measurements.

The ellipsometry measurements are carried out using a pulsed laser,which is disadvantageous under normal conditions, since the bandwidth ofthe pulsed laser is much greater than that of a CW laser of a typenormally employed for ellipsometry measurements. The ellipsometrymeasurement capability is useful in performing certain of theembodiments of the method described below, wherein it is an advantage todetermine the index of refraction and thickness of one or more of thefilm layers disposed over the substrate.

Referring to FIG. 6, if transient optical measurements are being made,the rotating compensator 132 is usually oriented such that the probebeam is linearly polarized orthogonal to the pump beam. This is toreduce the amount of scattered pump light that can reach the detector ofthe reflected probe beam. As will be seen below, there may be samplesfor which there is an advantage to having the probe and pump beams withthe same polarization. An analyzer 134 may be embodied as a fixedpolarizer, and also forms a portion of the ellipsometer mode of thesystem. When the system is used for transient optical measurements theanalyzer 134 is oriented to block the pump.

When used in the ellipsometer mode, the analyzer 134 is oriented so asto block light polarized at 45 degrees relative to the plane of theincident and reflected probe beam.

The embodiment of FIG. 6 further includes a dichroic mirror (DM2), whichis highly reflective for light in a narrow band near the pumpwavelength, and is substantially transparent for other wavelengths.

It should be noted in FIG. 6 that BS4 is moved to sample the pump beamin conjunction with BS3, and to direct a portion of the pump to D3 andto a second PSD (PSD2). PSD2 (pump PSD) is employed in combination withthe processor 101, computer controlled stage 122 (tilt and z-axis), andPSD1 (probe PSD) to automatically focus the pump and probe beams ontothe sample to achieve a desired focusing condition. Also, a lens L1 isemployed as a pump, video, and optical heating focusing objective, whilean optional lens L6 is used to focus the sampled light from BS5 onto thevideo camera 124.

Reference is now made to FIG. 7 for illustrating an embodiment ofapparatus 104, specifically a single wavelength, normal pump, obliqueprobe, combined ellipsometer embodiment. As before, only those elementsnot described previously will be described below. Shutter 1 and shutter2 are computer controlled shutters, and allow the system to use a He—Nelaser 136 in the ellipsometer mode, instead of the pulsed probe beam.For transient optical measurements shutter 1 is open and shutter 2 isclosed. For ellipsometer measurements shutter 1 is closed and shutter 2is opened. The He—Ne laser 136 is a low power CW laser, and has beenfound to yield superior ellipsometer performance for some films.

FIG. 8 is a dual wavelength embodiment 106 of the system illustrated inFIG. 7. In this embodiment the beam splitter 126 is replaced by aharmonic splitter 138, an optical harmonic generator that generates oneor more optical harmonics of the incident unsplit incident laser beam.This is accomplished by means of lenses L7, L8 and a nonlinear opticalmaterial (DX) that is suitable for generating the second harmonic fromthe incident laser beam. The pump beam is shown transmitted by thedichroic mirror (DM1 138 a) to the AOM1, while the probe beam isreflected to the retroreflector. The reverse situation is also possible,i.e., the shorter wavelength may be transmitted, and the longerwavelength may be reflected, or vice versa. In the simplest case thepump beam is the second harmonic of the probe beam (i.e., the pump beamhas one half the wavelength of the probe beam). It should be noted thatin this embodiment the AOM2 can be eliminated and instead a color filter(not shown) can be used in front of the detector Dl in order to reducethe amount of pump light reaching the detector Dl. The color filter isrequired to have high transmission for the probe beam and the He—Newavelengths, but very low transmission for the pump wavelength.

Finally, FIG. 9 illustrates a normal incidence, dual wavelength,combined ellipsometer embodiment 108. In FIG. 9 the probe beam impingeson PBS2 and is polarized along the direction which is passed by thePBS2. After the probe beam passes through WP3, a quarter wave plate, andreflects from the sample, it returns to PBS2 polarized along thedirection which is highly reflected, and is then directed to a detectorD0 in detector block 130. D0 measures the reflected probe beamintensity.

In greater detail, WP3 causes the incoming plane polarized probe beam tobecome circularly polarized. The handedness of the polarization isreversed on reflection from the sample, and on emerging from WP3 afterreflection, the probe beam is linearly polarized orthogonal to itsoriginal polarization. BS4 reflects a small fraction of the reflectedprobe onto an Autofocus Detector AFD.

DM3, a dichroic mirror, combines the probe beam onto a common axis withthe illuminator and the pump beam. DM3 is highly reflective for theprobe wavelength, and is substantially transparent at most otherwavelengths.

D1, a reflected He—Ne laser 136 detector, is used only for ellipsometricmeasurements.

It should be noted when contrasting FIG. 9 to FIGS. 7 and 8, that theshutter 1 is relocated so as to intercept the incident laser beam priorto the harmonic splitter 138. Based on the foregoing descriptions, aselected one of these presently preferred embodiments of measurementapparatus provide for the characterization of samples in which a shortoptical pulse (the pump beam) is directed to an area of the surface ofthe sample, and then a second light pulse (the probe beam) is directedto the same or an adjacent area at a later time. The retroreflector 129in all of the embodiments of FIGS. 3, 6, 7, 8 and 9 can be employed toprovide a desired temporal separation of the pump and probe beams. FIG.10 illustrates various time delays (t_(D)) between the application of apump beam pulse (P1) and a subsequent application of a probe beam pulse(P2), for times ranging from t₁ to t_(MAX).

If the sample includes a periodic array of structures disposed over asurface, the sample can act as a diffraction grating. Consequently, forsome range of wavelength of the probe light there is a diffractedcomponent, or components, to the reflected probe beam. Let R_(diff) bethe ratio of the power of one particular diffracted component of theprobe light to the power of the incident probe beam, and let ΔR_(diff)(t) be the transient change in R_(diff) induced by the application ofthe pump beam. For some samples it may be advantageous to measureΔR_(diff) (t), rather than the change in the strength of the specularlyreflected probe beam. This measurement can be made through the use of asecond detector of reflected probe light that can be moved undercomputer control to a position so as to receive the diffracted componentof the reflected probe beam. The position of this detector is determinedby the following parameters: a) the spacing between the structuresdisposed on the surface of the sample; b) the wavelength of the incidentprobe light; and c) the angle of incidence of the probe light.

The five embodiments 100, 102, 104, 106 and 108, as described above,have in common the feature that a sequence of pump pulses are generatedand directed at the surface of the sample. Each pump pulse illuminatesthe same area of the sample with an intensity that varies smoothlyacross the area. It is also within the scope of this invention to makemeasurements of the transient optical response by means of the inducedtransient grating method. See: D. W. Phillion, D. J. Kuizenga, and A. E.Siegman, Appl. Phys. Lett. 27, 85 (1975).

To induce a transient grating each pump pulse is divided into two ormore components by means of a beam splitter or beam splitters, thesecomponents then pass through separate optical paths, and are then alldirected onto the same area of the surface of the sample. If thedifferent components are directed onto the surface with different anglesthere are places within the area where the different componentsinterfere constructively and places where the interference isdestructive. Thus the total intensity of the pump light varies acrossthe sample surface.

In the case that only two components 201 and 201′ are present, as shownin FIG. 11, the intensity varies periodically across the sample surface.The periodicity of the intensity, i.e., the spacing between successivepoints of maximum intensity, is determined by the wavelength of the pumplight and the angles at which the different components of the pump lightare incident onto the surface. As a result of this periodic variation inthe intensity, the amount of pump light absorbed in each structure isnot the same, and also the amount of pump light absorbed in the filmsand the substrate varies periodically across the surface of the sample.The amplitude of the strain pulses that are generated thus variesperiodically across the sample. Consequently, the transient changes inthe optical properties of the sample, which result from the propagationof these strain pulses, also vary periodically. This variation of thetransient changes in the optical properties of the sample is equivalentto the production of a transient diffraction grating coinciding with thesample surface. When probe light 202 is incident on the area excited bythe pump, a part 204 of the probe light is diffracted, i.e., a part ofthe probe light is reflected in a direction, or directions, away fromthe direction 203 of specular reflection. Measurement of the intensityof this diffracted probe light by means of the detector D1 as a functionof the time delay t between the application of the pump and probe beamsprovides an alternate method for the characterization of the transientoptical response produced in the sample. Note that this mechanism forproduction of a diffracted probe beam is dependent on the generation ofa periodic variation in the intensity of the pump beam, whereas thediffracted probe beam considered in the preceding section originatesfrom the periodic arrangement of the structures on the sample surface.Furthermore, the use of the transient grating to determine the transientoptical response of the sample can be employed in the variousembodiments of measurement techniques described below for use withsamples that include substructures.

Typical characteristics of the light pulses employed in the systems 100,102, 104, 106, and 108, of FIGS. 3, 6, 7, 8 and 9, respectively, are asfollows. The pump pulse has an energy of approximately 0.001 to 100 nJper pulse, a duration of approximately 0.01 psecs to 100 psec per pulse,and a wavelength in the range 200 nm to 4000 nm. The pulse repetitionrate (PRR) is in the range of 100 Hz to 5 Ghz and, as is shown in FIG.12, the pump pulse train may be intensity modulated at a rate of 1 Hz to100 MHz, depending on the PRR. The pump pulse is focused to form a spoton the sample surface of diameter in the range of approximately 10micrometers to 20 micrometers, although smaller spot sizes, and hencebetter lateral resolution can also be employed.

Referring to FIG. 13, it is also within the scope of the teaching ofthis invention to deliver the pump pulse, or the probe pulse, or boththe pump and probe pulses, through an optical fiber 244. Alternatively,a second input fiber 246 can be provided, whereby the pump pulse isdelivered through the fiber 244 and the probe pulse is delivered throughthe fiber 246. Another fiber 248 can also be employed for receiving thereflected probe pulse and delivering same to the photodetector (notshown). For this embodiment the ends of the optical fiber(s) are affixedto and supported by a holding stage 250. The holding stage 250 ispreferably coupled through a member 252 to an actuator 254, such as alinear actuator or a two-degree of freedom positioning mechanism. Inthis manner the reliability and repeatability of the measurement cycleis improved, in that the size and position of the focused pump, probe,or pump and probe beams on the sample surface are independent of minorchanges in the direction or profile of the laser output beams, orchanges in the profile of the probe beam associated with the motion ofany mechanical stage that may be used to effect the delay t. Preferably,the angular orientation between the end of the probe beam delivery fiberand the end of the reflected probe beam fiber is such as to optimize thegathering of reflected probe beam light from the sample surface. It isalso within the scope of this invention to use one or more lensesfollowing the fiber or fibers, in order to focus the output beams fromthe fibers onto the sample surface, or to collect the reflected probelight and to direct it into the fiber 248 of FIG. 13.

FIG. 14 shows an embodiment wherein a terminal portion 244 b of a pumpand/or probe beam delivery fiber 244 a is reduced in diameter, such asby stretching the fiber, so as to provide a focused spot 244 c having adiameter that is less than the normal range of optical focusing. Whencoupled with the embodiment of FIG. 13 this enables the pump and orprobe optical pulse to be repeatably delivered to a very small region ofthe sample surface, e.g., to a spot having a diameter <one micrometer,regardless of any changes that are occurring in the optical path lengthof the probe beam.

It is also within the scope of the invention to measure other transientoptical responses instead of the change in the optical reflectivity. Aspreviously mentioned, the apparatus 100, 102, 104, 106, and 108, asshown in FIGS. 3, 6, 7, 8 and 9, respectively, are capable of measuringthe (1) transient change in the reflectivity ΔR(t) of the probe beam.With suitable modifications, the apparatus can be used to measure (2)the change ΔT in the intensity of the transmitted probe beam, (3) thechange ΔP in the polarization of the reflected probe beam, (4) thechange Δφ in the optical phase of the reflected probe beam, and/or (5)the change in the angle of reflection Δσ of the probe beam. Thesequantities may all be considered as transient responses of the samplewhich are induced by the pump pulse. These measurements can be madetogether with one or several of the following: (a) measurements of anyor all of the quantities (1)-(5) just listed as a function of theincident angle of the pump or probe light, (b) measurements of any ofthe quantities (1)-(5) as a function of more than one wavelength for thepump and/or probe light, (c) measurements of the optical reflectivitythrough measurements of the incident and reflected-average intensity ofthe pump and/or probe beams; (d) measurements of the average phasechange of the pump and/or probe beams upon reflection; and/or (e)measurements of the average polarization and optical phase of theincident and reflected pump and/or probe beams. The quantities (c), (d)and (e) may be considered to be average or static responses of thesample to the pump beam.

The measured results for ΔR(t), or other transient optical response, canbe compared with simulations of the propagation of strain pulses in thesample. A complete simulation can be performed by the following steps:

a) The sample is described by a number of physical parameters, includingbut not limited to, the dimensions of each structure, the spacingbetween the structures, the thickness of any films making up the sample,the electrical resistivity of the sample material, etc. The electricalresistivity is a significant parameter because it affects the way strainpulses are generated in the sample as a result of the absorption of thepump pulse.)

b) The absorption of the pump light pulse is then considered, and thechange in temperature of each part of the sample is determined.

c) The thermal stress that results from this temperature change iscalculated, and the amplitude of the generated strain pulses isdetermined.

d) The location of these strain pulses as a function of the time t iscalculated and the time-dependent strain distribution in the sample isfound.

e) From this strain distribution the change in optical reflectivity, orother transient optical response, is calculated and compared with themeasured result.

f) The parameters of the sample are adjusted so as to obtain a best fitwith the measured data.

For some samples, the available information may be insufficient to makean analysis of this type. In such samples a more limited approach may beused to obtain information about selected parameters of the sample.

For example, FIG. 15 shows a sectional view of a two-dimensionallypatterned sample composed of an array of structures, i.e. wires, eachwith rectangular cross-section embedded in a substrate 305. The pump andprobe beams are directed at oblique incidence, i.e., neitherperpendicular nor parallel to the surface of the sample. Absorption ofthe pump beam on the side wall 303 of each structure 301, generates astrain pulse 307 that propagates across the structure 301 and causes achange in the reflection of the probe beam. To model this particularcontribution to ΔR(t), it may be sufficient to use a simplifiedapproach. Since the side walls of the structure 301 are parallel, ameasurement of the arrival time of the strain pulse is sufficient todetermine the width of structure 301.

In the event that the side walls are at a non-normal angle, as is shownfor structure 310 in FIG. 16, the strain pulse is broadened becausestrain generated at different locations 314 and 316 on the side wall 311of structure 310 travels a different distance to reach the far side ofstructure 310. Thus, an echo feature in ΔR(t) is broadened by an amountthat increases with the angle of side wall 311.

A second acoustic pulse 318 is generated at the top surface of structure310 and travels in the direction perpendicular to the plane 312 ofsubstrate 315. This gives rise to a separate series of echoes whosespacing in time can be used to measure the height b of structure 310.

It is also within the scope of this invention to detect echoes arisingfrom the part of the strain pulse that is reflected at boundaries withina structure. For example, in FIGS. 17 and 18 each structure includes awire 320 of one material with a liner 325 of another material, e.g., anoxide, on each side. Pump light that is absorbed on the sides of thewire 320 generates a strain pulse at the surface 330 of liner 325. Thisstrain pulse is partially reflected at the interface between the liner325 and the core material of the wire 320. The part of the strainreturning directly to the outer surface of the liner 325 gives rise toan echo feature. From the time at which this echo occurs, the thicknessof liner 325 can be found.

It is also within the scope of the invention to make measurements onsamples composed of structures with dimensions so small that the spatialextent of the generated strain pulse is comparable to the thickness, orwidth, of the structure. For such samples it is not as useful toconsider that the generated strain pulse bounces back and forth withinthe sample. Instead, one should consider that the pump pulse exciteseach structure into one or more of its normal modes of vibration. Underthese conditions, the change in optical reflectivity ΔR(t) varies withtime t as a sum of a number of oscillatory components with differentfrequencies and damping rates. These frequencies and damping rates canbe determined from the measured ΔR(t) by the following methods:

(a) The Fourier transform of ΔR(t) is taken. Peaks in the Fourierspectrum are identified with the normal mode frequencies. The widths ofthe peaks can be used to give the damping rates of the individual normalmodes.

(b) The measured ΔR(t) can be fit to a sum of damped oscillations withdifferent frequencies. This fitting process can be accomplished throughthe use of a standard non-linear least squares fitting algorithm.

Other analysis methods will be apparent to those skilled in the art,when guided by the foregoing teachings in accordance with the presentinvention.

The results for the frequencies and damping rates can then be comparedwith frequencies and damping rates obtained from a computer simulationof the vibrations of the sample. In more detail:

a) The sample is described by a number of physical parameters, includingbut not necessarily limited to, the dimensions of each structure, thespacing between the structures, the thickness of any films making up thesample, etc.

b) The frequencies and the damping rates are calculated using, forexample, a finite-element simulation of the vibrations of the structure.

c) Steps (a) and (b) are repeated with each physical parameter variedover a suitable range.

d) The measured frequencies and damping rates are compared with thecalculated frequencies and damping rates obtained for each set ofparameters, and the set of parameters that gives frequencies and dampingrates closest to those measured is determined.

Variations of this method may include, but are not limited to:

i) Use of methods other than finite-element simulation to calculate thefrequencies and damping rates. For example, a molecular dynamicsapproach may be more suitable for some samples.

ii) The method as described above amounts to the establishment of acatalog of frequencies and damping rates for a range of physicalparameters. An alternate method starts from some initial set of physicalparameters, compares the frequencies and the damping rates to themeasured frequencies and damping rates, and then repeatedly adjusts thephysical parameters so as to improve the agreement between simulationand experiment, until a best fit is obtained.

iii) Use of the amplitude of the contributions from the different modesto give information about the physical parameters of the sample. Forexample, the modes that have a large strain amplitude in the regionswhere the pump pulse is strongly absorbed will have a large amplitude.

The list of physical parameters can include the adhesion of one part ofeach structure to another, the adhesion between the structure and thefilm or films in which it is embedded or disposed upon, and the soundvelocity and density of the different components of the sample.

For some samples, it is advantageous to make measurements withparticular choices of the angle of incidence of the pump and/or theprobe beam. There may be advantages to particular choices of thepolarization of these beams. In addition, it may be advantageous tomeasure the sample for more than one selection of the angles ofincidence and polarization. The reasons for this include:

a) Achievement of a better signal to noise ratio. This can reduce thetime for the measurement to be made.

b) Enhancement of some features of particular interest relative to otherfeatures that are readily apparent in the measured ΔR(t), but which areless important. For example, in FIG. 17 is shown a two-dimensionallypatterned sample composed of an array of wires embedded in a substrate.Each wire 320 has a liner 325 in the form of a thin slab of a differentmaterial on its sides. If it is desired to measure the thickness of theliner 325 it may be favorable to direct the pump beam in a direction inthe x-y plane at oblique incidence to the sample surface so that a largepart of the beam is absorbed in liner 325. This enhances the amplitudeof echoes arising from a strain pulse bouncing back and forth in liner325. The amount of energy absorbed in the liner 325 may also beincreased through appropriate choice of the polarization of the pumpbeam. The magnitude of the echo appearing in ΔR(t) due to the strainpulse in the coating may be further enhanced through a suitable choiceof the direction of the probe beam and by choice of the polarization ofthe probe beam.

c) Simplification of the identification of the vibrational modes. Anessential part of the method described above is the identification ofindividual measured mode frequencies with frequencies of modes obtainedby the simulation. To aid in this identification process it is helpfulfor some samples to take advantage of the symmetry of the normal modes.For example, in FIG. 18 is shown the same sample as is shown in FIG. 17,but now the pump beam is directed at normal incidence. For this samplethe y-z plane is a plane of mirror symmetry. It is clear that whenexcited in this way only modes for which the strain is an even functionof x are excited (even modes). For these modes the displacement is anodd function of x. Thus, the measured ΔR (t) includes a sum ofoscillations whose frequencies should be compared only with thefrequencies of the modes from the simulation that have the samesymmetry, i.e., even modes. Thus, one possible procedure is thefollowing:

i) Measure the sample with a pump beam at normal incidence as shown inFIG. 18, and determine the frequencies and damping rates of the normalmodes (even symmetry modes).

ii) Identify these modes with modes of even symmetry obtained from thesimulation.

iii) Change the orientation of the pump beam to oblique incidence, anddetermine a new set of frequencies and damping rates. This set containsfrequencies not present in the set obtained in i). These frequencies arelikely to correspond to modes with odd symmetry (strain an odd functionof x).

For some samples the same advantages just described may also ensue frommeasurement of changes in the diffracted probe beam. Correspondingadvantages may also result from the use of the transient grating method.

For laterally-patterned samples in which the structures form an array,it is also within the scope of this invention to use the angle ofdiffraction of the pump and/or the probe beam to deduce the repeatdistance of the array of structures. It is also within the scope of thisinvention to supply a separate light beam, e.g., a He—Ne laser, to thesample, to measure the angle of diffraction of this beam, and tocalculate the repeat distance of the array from this diffraction angle.This repeat distance can be used as an input to the analysis.

It is also within the scope of this invention to use a combination ofsimulations of propagating strain pulses and normal mode analysis. Forexample, the part of ΔR(t) corresponding to early times may show sharpfeatures best analyzed in terms of echoes due to strain pulses, whilethe part of ΔR(t) at longer times may be better described in terms ofnormal modes.

It is within the scope of this invention for the pump and probe beams tobe focused so that they illuminate a large number of the similarstructures that make up the sample, or just a few of these structures.In the event that only a few structures are illuminated it may not bepossible to detect a diffracted probe beam.

In some samples there may be cracks or voids or other mechanical defectswithin one element of the structure, as distinct from poor adhesion of astructure element to the film, or films, in which it is embedded. Suchdefects may make a large change in the frequency and damping rate ofcertain of the normal modes. From the determination of which modes areaffected and the extent of the changes in frequency and damping, it ispossible to determine the location and size of the defect. For example,the effect of a defect on a mode is large if it is at a position in thesample where the oscillating strain due to the normal mode is large. Itis also within the scope of this invention to identify such defects bycomparison with measurements on samples known to contain certaindefects.

In addition to the contribution to the change in reflectivity from thepropagating strain pulses, there is a contribution to the measured ΔR(t)from temperature changes induced in the sample by the application of thepump light pulse. A change in temperature results in a change in theoptical constants of a material, this effect being referred to asthermo-reflectance. The thermo-reflectance contribution to ΔR(t) is asmoothly varying function of time, and is thus readily distinguishablefrom the contribution due to the strain pulses which includes a seriesof echoes or oscillations. For some samples, from an analysis of thethermo-reflectance contribution to ΔR(t) it is possible to determine therate of change in temperature of different parts of the sample. Fromthis rate of change, it may be possible to estimate the thermalconductivity of one or more of the elements making up the sample, or todetermine the Kapitza conductance at one or more of the interfacesbetween the different components of the sample, for example, betweeneach structure and the film or films in which it is embedded or disposedupon. The Kapitza conductance is enhanced at interfaces where thematerials are in intimate contact, and thus can be used as a measure ofthe adhesion at an interface.

If the structures making up a laterally patterned sample have avariation in their dimensions, the frequency of the normal modes variesfrom structure to structure. When some number of these structures areset into vibration by the pump light pulse, the vibrations in eachstructure is initially in phase. As time progresses, however, thevibrations become out of phase, resulting in an increased damping of theoscillation appearing in the measured ΔR(t). It is within the scope ofthis invention to use a measurement of this increased damping rate tocharacterize the variation in the dimensions of the structures withinthe measurement region.

FIG. 19 is a flowchart of one method for characterizing a sample inaccordance with the present invention. The sample includes a structuredisposed on or within the sample.

In step 510, a first pulse of light is applied to a surface of thesample for creating a propagating strain pulse in the sample.

In step 520, a second pulse of light is applied to the surface so thatthe second pulse of light interacts with the propagating strain pulse inthe sample.

In step 530, a change in optical response of the sample is sensed from areflection of the second pulse.

In step 540, a time of occurrence of the change in optical response isrelated to at least one dimension of the structure.

FIG. 5 is a flowchart of a second method for characterizing a sample inaccordance with the present invention. The sample includes a structuredisposed on or within the sample.

In step 610, a first pulse of light is applied to a surface of thesample to excite the structure into a vibration in at least one of itsnormal modes.

In step 620, a second pulse of light is applied to the surface.

In step 630, a change in optical response of the sample is sensed from areflection of the second pulse.

In step 640, the change in optical response is related to an oscillatorycomponent of the vibration.

In step 650, the oscillatory component is associated to at least one ofa spatial or electrical characteristic of the structure.

The present invention can be used to characterize structures made of anymetal or metal alloy including copper, cobalt, titanium, aluminum, gold,nickel, silver, tungsten, etc. The invention can also be used tocharacterize polysilicon gate structures, and polysilicon gatestructures onto which a metal contact layer has been deposited. Thesample can be any semiconductor material, for example, a Group IVAsemiconductor, a Group IIB-VIA semiconductor (e.g., HgCdTe, InSb), aGroup IIIA-VA semiconductor (e.g., GaAs, GaAlAs), or combinationsthereof. In addition, the sample may comprise a desirednon-semiconductor layer, such as a substrate or an overlayer, comprisedof a glass, sapphire or diamond.

While the invention has been particularly shown and described withrespect to preferred embodiments thereof, it will be understood by thoseskilled in the art that changes in form and details may be made thereinwithout departing from the invention. Accordingly, the present inventionis intended to embrace all such alternatives, modifications andvariances that fall within the scope of the appended claims.

1-61. (canceled)
 62. A method for characterizing a sample having aplurality of structures disposed on or within said sample, comprisingthe steps of: applying a first pulse of light to a surface of saidsample for creating a propagating strain pulse in said sample; applyinga second pulse of light to said surface so that said second pulse oflight interacts with said propagating strain pulse in said sample;sensing from a reflection of said second pulse a change in opticalresponse of said sample; and relating a time of occurrence of saidchange in optical response to at least one dimension of said structure,wherein said step of relating includes the steps of: applying a beam oflight to said surface; determining an angle of diffraction of said beamof light; and calculating a repeat distance of said plurality ofstructures based on said determined angle of diffraction.
 63. A methodas in claim 62, further comprising comparing said time of occurrence toa result of a computer simulation of a propagation of said strain pulsein said sample.
 64. A method as in claim 62, wherein at least one ofsaid first pulse and second pulse is applied at oblique incidence.
 65. Amethod as in claim 62, wherein at least one of said first pulse andsecond pulse is applied with a predetermined polarization.
 66. A methodas in claim 62, wherein said structure comprises a metal or metal alloy.67. A method as in claim 62, wherein said sample comprises asemiconductor material.
 68. A method as in claim 62, wherein said samplecomprises at least one layer of a non-semiconductor material.
 69. Amethod as in claim 62, further comprising moving a detector to receivesaid diffracted component of said beam of light.
 70. A method as inclaim 62, further comprising applying a second light beam to the sampleand determining an angle of diffraction of said second beam of light,wherein calculating a repeat distance of said plurality of structures isfurther based upon said angle of diffraction of said second beam oflight.
 71. A method as in claim 62, wherein said at least one dimensionis a width of said structure.
 72. A method as in claim 62, furthercomprising performing at least one ellipsometry measurement of thesample.
 73. A system for characterizing a sample having a structuredisposed on or within said sample, comprising: an optical source unitconfigured to apply a first pulse of light to a surface of said samplefor creating a propagating strain pulse in said sample, and to apply asecond pulse of light to said surface so that said second pulse of lightinteracts with said propagating strain pulse in said sample; whereinsaid optical source unit is further configured to apply a beam of lightto said surface; a sensor configured to sense a reflection of saidsecond pulse a change in optical response of said sample; and aprocessor configured to relate a time of occurrence of said change inoptical response to at least one dimension of said structure, whereinsaid processor is further configured to determine an angle ofdiffraction of said beam of light and calculate a repeat distance ofsaid plurality of structures based on said determined angle ofdiffraction.
 74. A system as in claim 73, wherein said processor isfurther configured to compare said time of occurrence to a result of acomputer simulation of a propagation of said strain pulse in saidsample.
 75. A system as in claim 73, wherein at least one of said firstpulse and second pulse is applied at oblique incidence.
 76. A system asin claim 73, further comprising at least one polarizer, wherein at leastone of said first pulse and second pulse is polarized with said at leastone polarizer to have a predetermined polarization.
 77. A system as inclaim 73, further comprising a detector configured to be moved toreceive said diffracted component of said beam of light.
 78. A system asin claim 73, further comprising an ellipsometer.
 79. A non-destructivesystem for characterizing a sample having a structure disposed on orwithin said sample, comprising: a first pulse applying means forapplying a first pulse of light to a surface of said sample for creatinga propagating strain pulse in said sample, a second pulse applying meansfor applying a second pulse of light to said surface so that said secondpulse of light interacts with said propagating strain pulse in saidsample; a beam applying means for applying a beam of light to saidsurface; a sensing means for sensing from a reflection of said secondpulse a change in optical response of said sample; and a relating meansfor relating a time of occurrence of said change in optical response toat least one dimension of said structure, wherein said relating means isfurther for determining an angle of diffraction of said beam of light;and calculating a repeat distance of said plurality of structures basedon said determined angle of diffraction.
 80. A system as in claim 79,wherein said first pulse applying means, second pulse applying means andbeam applying means is an optical source unit; said sensing means is asensor; and said relating means is a processing unit.
 81. A system as inclaim 79, further comprising a means for making an ellipsometrymeasurement.